FIG. 1 shows a traditional differential signaling scheme. As observed in FIG. 1, a variable “x” is driven from the transmission side 101 to the reception side 102 as a differential signal having a positive polarity (+x) along a first channel 103a and a negative polarity (−x) along a second channel 103b. In the electronic arts, for bit stream variables at least, differential signals have been used to transport a variable at a high speed. Here, having both a positive polarity (+) and a negative polarity (−) boosts the signal-to-noise ratio (SNR) over and above a “single ended” transmission of the same variable (i.e. where just one sense is employed), while, at the same time reducing emitted electro-magnetic interference (EMI).
Qualitatively, the boost in SNR can be viewed as a consequence of the fact that two channels are used to transmit the same variable. In a manner of speaking, the “signal power” of the transmitted variable is increased. Because a minimally acceptable bit error rate (BER) is typically defined at a minimally acceptable SNR, and, because SNR degrades as the fundamental frequency of the transmission of the variable increases, the boost in SNR from the differential signaling corresponds to a higher achievable fundamental frequency of transmission for the variable (e.g., a higher bit rate speed).
FIG. 2 shows an arrangement of multiple differential signals to transport a plurality of variables x, y, z. Multiple differential signals have been traditionally used to implement a high speed bus or high speed interface where multiple bit streamed variables x, y, z are to be simultaneously transported from a same transmission side 201 to a same reception side 202. The arrangement of FIG. 2 presents some problems, however.
A first problem is the number of channels needed to fully implement the high speed bus/interface. Specifically, 2N channels are needed to transport N variables. In the example of FIG. 2, N=3 (x, y and z correspond to three variables). Therefore there are 2N=2(3)=6 channels: 203a,b; 204a,b; 205a,b. In the electronic arts, each of the 2N channels corresponds to individual wires that must be routed from the transmission side 201 to the reception side 202. If N is large, the designer of the bus/interface is presented with an expensive (at least in terms of needed layout space) if not impossible wire routing challenge.
A second problem is that the SNR boost from the differential arrangement can be less than that for a single differential channel as described above with respect to FIG. 1. Here, if the respective channels for the multiple variables are proximate to one another (e.g., neighboring one another as observed in FIG. 2), which is apt to be the case if a large N has caused the designer to “pack” the individual channel wires close to one another, then, according to a phenomena known as “crosstalk”, the signals associated with different uncorrelated variables can act as noise sources to each other. For example, the signals for variables x and z on channels 203a,b and 205a,b can act as noise on the channels 204a,b for variable y. The added noise effectively reduces the SNR boost described above for the differential scheme of FIG. 1.